linux-kernel - Re: hotplug support for arch/arc/plat-eznps platform

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Date:   Mon, 7 Aug 2017 10:33:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Ofer Levi(SW)" <oferle@...lanox.com>
Cc:     "rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
        "vatsa@...ibm.com" <vatsa@...ibm.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: hotplug support for arch/arc/plat-eznps platform

On Sun, Aug 06, 2017 at 05:53:37AM +0000, Ofer Levi(SW) wrote:
> I'm adding hot-plug support for the above arch and ran into performance issue with execution of 
> partition_sched_domains ()  - About 0.5 sec per cpu, which is unacceptable with the arch supported 4k cpus.
> To my limited understanding, on the plat-eznps arch, where each cpu is always running a single threaded process in 
> isolation mode, the above call is not necessary,
> Can you please confirm (or reject) my understanding? If I'm wrong short explanation will be appreciated.
> If I'm correct, I will be grateful if you can point me to how to skip this call in a way acceptable by community.

You've failed to explain why you think hotplug should be a performance
critical path.

I'm also not seeing how it would be different from boot; you'd be
looking at a similar cost for SMP bringup.